Cellulose-based foodstuff casings prepared by the amine oxide process

ABSTRACT

The invention concerns flat or tubular cellulose-based foodstuff casings which are prepared by extruding (“spinning”) cellulose dissolved in N-methyl-morpholine-N-oxide by means of a nozzle with an annular gap. After extrusion, the casings are stretched transversely in ambient air by blow moulding, and are then treated with an NMMO-containing aqueous spinning bath. The tubular casings are particularly suitable as sausage casings. Cut open, they can also be used as flat foils.

[0001] The invention relates to flat or tubular cellulose-based foodcasings which are produced by extruding (“spinning”) cellulose dissolvedin N-methylmorpholine N-oxide. The casings are particularly suitable assausage casings.

[0002] Cellulose is insoluble in the usual solvents. It does not have amelting point or melting range and cannot therefore be melt-processedeither. Therefore, it is usually chemically modified for producing foodcasings. However, these processes are associated with a breakdown of thecellulose, i.e. the mean degree of polymerization of the cellulosebecomes lower. In addition, the processes are highly technically complexand correspondingly expensive.

[0003] Currently, the viscous process is preferred. In this process, thecellulose is reacted with sodium hydroxide solution and then with carbondisulfide. This produces a yellow-orange-colored cellulose xanthogenatesolution which is extruded through a spinneret. The cellulose is thenregenerated using precipitation and washing baths. A variety ofapparatuses have had to be developed for this for cleaning up exhaustair and wastewater.

[0004] As early as 1936 it was discovered that cellulose is soluble inoxides of tertiary amines (DE 713 486); however, this discovery was notpursued further until 30 years later. In the course of this,N-methylmorpholine N-oxide (NMMO) was identified as the most suitablesolvent. The cellulose dissolves therein without being chemicallymodified. No breakdown of the cellulose chains takes place. Preparationof the corresponding spinning solutions has also been disclosed (DD 218104; DD 298 789; U.S. Pat. No. 4 145 532; U.S. Pat. No. 4 196 282; U.S.Pat. No. 4 255 300). Yarns may be produced from the solutions byextrusion into a spinning bath (DE-A 44 09 609; U.S. Pat. No. 5 417909). WO 95/07811 (=CA 2 149 218) also discloses a process for producingtubular cellulose films by the amine oxide process. A characteristic ofthis process is cooling the extruded film by cooling gas immediatelybelow the annular gap of the extrusion die. According to EP-A 662 283,the extruded tubular film is cooled internally using liquid.

[0005] Recovery and purification of the NMMOs are described in DD 274435. Since the cellulose is not chemically modified in the process, lessequipment is required. In the amine oxide process, no gaseous or aqueouswaste products are produced, so that there are no problems with respectto the exhaust air or the wastewater. It is therefore achievingincreasing importance.

[0006] EP-A 0 686 712 describes the production of flexible cellulosefibers by the N-methylmorpholine N-oxide (NMMO) spinning process. Inthis process, a cellulose solution in aqueous NMMO is forced through aspinneret, conducted via an air section into an NMMO-containing aqueousprecipitation bath and then washed, post-treated and dried.

[0007] According to WO 93/13670, a seamless tubular food casing isproduced by extruding a solution of cellulose in NMMO/water using aspecial extrusion die. An air section is situated between extrusion dieand precipitation bath. A characteristic of this process is a speciallyshaped hollow mandrel through which the precipitation liquid can alsocirculate in the interior of the tube. In the air section, the interiorof the extruded tube is virtually completely filled by a hollow mandreland precipitation liquid. The film is not stretched transversely in thecourse of this.

[0008] WO 95/35340 describes a process for producing blown cellulosefilms in which an underivatized cellulose dissolved in NMMO is used.

[0009] However, the amine oxide process also has disadvantages. Theunderivatized cellulose molecules are already preorientated in the NMMOsolution and are substantially more tightly packed than is the case withchemically modified (“derivatized”) molecules. On extrusion, theorientation in the longitudinal direction is still more pronounced. Theyarns thus produced therefore exhibit a high strength in thelongitudinal direction, but only low strength in the transversedirection. They have a strong tendency to split on being mechanicallystressed in the wet state. Films or other shaped bodies, which must beable to be loaded in the longitudinal and transverse direction, may thusscarcely be produced by this method.

[0010] The object was therefore to modify the amine oxide process insuch a manner that sufficiently load-bearing films or shaped bodies, inparticular tubular food casings, can be produced. The process shouldsucceed in this case with as few steps as possible, and should remaininexpensive and environmentally compatible.

[0011] The object can be achieved if the wet treatment is combined witha blow molding. The present invention thus relates to a seamless tubularcellulose-based film, which is obtainable by extruding a cellulose-,N-methyl-morpholine N-oxide- and water-containing spinning solutionthrough an annular die and treating the tubular film in anN-methylmorpholine N-oxide-containing aqueous spinning bath, whichcomprises the film being longitudinally stretched by blow molding in anair section between annular gap and surface of the spinning bath.

[0012] The spinning solution preferably comprises 7 to 15% by weight,particularly preferably 9 to 12% by weight, cellulose, in each casebased on the total weight of the spinning solution. The mean degree ofpolymerization of the cellulose in this case is preferably 300 to 700,particularly preferably 400 to 650. As solvent, the spinning solutionpreferably comprises 90.5 to 92.5% by weight NMMO and 9.5 to 7.5% byweight water. The parameters mentioned in this paragraph, together withthe temperature, essentially determine the viscosity and fluid behaviorof the spinning solution.

[0013] Processes for preparing the spinning solution are generallyfamiliar to those skilled in the art. Customarily, cellulose is mashedin a 60% strength by weight aqueous NMMO solution at room temperature.The cellulose usually originates from wood or cotton. As the temperatureincreases, water is then distilled off in a heated stirred tank underreduced pressure until the residue consists of cellulose and NMMOmonohydrate. This is the case at an NMMO content of 87.7% by weight,based on the total weight of NMMO and water. The ratio of NMMO to watermay be readily determined by the refractive index. In the NMMOmonohydrate, the cellulose dissolved completely at a temperature of 85to 95° C. with intensive stirring. The refractive index of the solutionis 1.4910 to 1.4930. The water content has decreased to 7.5 to 9.5% byweight. The spinning solution is degassed, filtered and transferred tothe spinning vessel.

[0014] Food casings having improved suppleness may be obtained ifmodifying compounds are added to the spinning solution. The compoundsmust be miscible with the cellulose/NMMO/water solution. The content ofthese compounds is generally 0.2 to 50% by weight, preferably 0.5 to 20%by weight, particularly preferably 1 to 15% by weight, in each casebased on the weight of the cellulose. The compounds may be mixedhomogeneously with the spinning solution at a temperature of 85 to 105°C., preferably 90 to 100° C.. Particularly suitable modifying compoundsare starch, starch derivatives and cellulose derivatives (in particularesters or ethers of the starch or cellulose), as well as sugar esters,and in addition hydrophilic naturally occurring polymers (preferablyalginic acid and alginates, chitosan and carrageenan). Suitablecompounds are also hydrophilic synthetic polymers (preferably vinylalcohol, vinyl acetates and acrylates) and polymers which simultaneouslypossess hydrophilic and hydrophobic properties (preferably esters from asugar-such as sucrose-and fatty acids, the esters having an HLB of 1 to15; HLB=hydrophilic-lipophilic balance). If appropriate, ethoxylatedfatty acids and salts thereof, for example stearic acid or calciumstearate, waxes and paraffins are also suitable. Finally,polyvinylpyrrolidone, copolymers of vinylpyrrolidone and2-(dimethylamino)ethylmethacrylate, copolymers of methyl vinyl ether andmaleic anhydride or of methyl vinyl ether and maleic acid monoalkylester may also be used. The modifying compounds may also becrosslinkable, as is the case with polyethyleneimines. These compoundsgenerally increase the suppleness, strength, clipping stability andshear stability of the shaped bodies according to the invention. Theyalso act as internal (primary) plasticizers. Impregnation with secondaryplasticizers (such as glycerol) can frequently be even entirely omittedif the content of the modifying compounds in the food casings accordingto the invention is great enough (generally of the order of magnitude 8%by weight or more, based on the weight of the dry cellulose).Furthermore, they generally decrease the tendency of the cellulose tocrystallize.

[0015] The spinning solution is extruded through the annular diepreferably at a temperature of 85 to 105° C., particularly preferably 90to 95° C. The annular gap is generally 0.1 to 2.0 mm wide, preferably0.2 to 1.0 mm. The width here must be adapted to the warpage. “Warpage”is defined as the quotient of the velocity on leaving the annular gap(exit velocity) and the velocity at which the extruded tube is taken off(take-off velocity). The warpage is generally 3.0 to 0.10, preferably2.0 to 0.2, particularly preferably 1 to 0.4. The exit velocity,depending on the construction of the plant, is 5 to 120 m/min,preferably 10 to 80 m/min. It is also determined by the caliber. On theextruded tube, advantageously, only a low tension is exerted in thelongitudinal direction, which is essentially due to its own weight.

[0016] The “air section”, i.e. the section between annular gap andsurface of the spinning bath in which the blow molding takes place, ispreferably 1 to 50 cm, particularly preferably 2.5 to 20 cm. It is alsodependent on the diameter (“caliber”) of the tubular film after the blowmolding. In contrast to the abovementioned WO 95/07811 and EP-A 662 283,no measures are required for additional cooling in the air section, andaccordingly they are also not provided. The extruded tube cools only asmall amount in the air section. Otherwise, transverse stretching wouldscarcely be possible. The blow molding is effected by compressed air orother gases which pass into the interior of the tube through orifices inthe die body. Stretching in the transverse direction considerablyincreases the transverse strength of the tube. Depending on warpage, thediameter of the blow-molded tube is up to 100% greater or up to 50%smaller, preferably up to 80% greater or up to 20% smaller, thanimmediately after exiting the annular gap. Transverse stretching with adiameter simultaneously becoming smaller is obviously only possible ifthe warpage is less than 1. Preferably, the diameter of the blow-moldedtube is 10 to 100% greater, particularly preferably 20 to 80% greaterthan immediately after exiting the annular gap.

[0017] If appropriate, the tube is conducted via a pipe, preferably ametal pipe. The diameter of this pipe can be selected between 30%greater and 30% smaller than that of the annular gap. Precipitationliquid and support air are fed via this pipe.

[0018] After entering the spinning bath, the diameter of the tubedecreases. Through appropriate apparatuses in the die body, the spinningbath solution also passes into the interior of the cellulose tube. As aresult, the tube solidifies more rapidly; at the same time, the insidesare prevented from sticking together. The liquid level in the interiorof the tube should not be significantly higher or lower than that of thesurrounding spinning bath. The spinning bath itself is an aqueoussolution which comprises 5 to 50% by weight, preferably 8 to 20% byweight, of NMMO. The temperature of the spinning bath is in the rangefrom 0 to 50° C., preferably 2 to 20° C.

[0019] The depth of the spinning bath is determined by the caliber ofthe cellulose tube, its wall thickness and the desired residence time inthe bath. Generally, the depth should be selected so that, on flatteningthe tube on the guide roll, the resulting edges are not damaged. In thecase of a tube of caliber 20, which, immediately after leaving theannular gap, has a wall thickness of 0.5 mm and passes through the bathat a velocity of 20 m per minute, the spinning bath has a depth of about3 m.

[0020] For further solidification, the laid-flat tube then passesthrough still more NMMO-containing precipitation vats. The firstprecipitation vat comprises approximately 10 to 20% by weight of NMMO.In the following precipitation vat, the NMMO content decreases. It hasbeen found to be favorable to increase the temperature from oneprecipitation vat to the next, up to about 60 to 70° C. in the last vat.The NMMO content in the tube is thus more greatly decreased.

[0021] This so-called “precipitation section” is followed bywater-filled wash vats, in which the last traces of NMMO are removedfrom the tube. The temperature of these baths is 15 to 70° C.,preferably 40 to 60° C.. Generally, a so-called plasticizer vat thenfollows. This comprises an aqueous solution of a plasticizer forcellulose. Suitable plasticizers are polyols and polyglycols, inparticular glycerol. The aqueous solution comprises 5 to 30% by weight,preferably 6 to 15% by weight, of plasticizer. The temperature of theplasticizer solution is advantageously 20 to 80° C., preferably 30 to70° C.. The glycerol content of the casing is then about 15 to 30% byweight, preferably 18 to 23% by weight, in each case based on its totalweight.

[0022] Thereafter, the tubes are conducted through a hot-air dryer inthe inflated state. Expediently, drying is performed at decreasingtemperature (from about 150° C. at the inlet to about 80° C. at theoutlet of the dryer). An additional transverse orientation may beachieved, if appropriate, by appropriately increased internal pressureon drying. Otherwise, the tube is inflated on drying to the originalcaliber, in order to retain the degree of transverse orientation onceachieved. During drying, the swelling value decreases to 130 to 180%,preferably 140 to 170%, depending on drying conditions and glycerolcontent. The tube is then wetted until the water content is 8 to 20% byweight, preferably 16 to 18% by weight, in each case based on the totalweight of the tube. Then, using a pinch-roll pair, it can be laid flatand wound up.

[0023] Used aqueous NMMO solution may be purified by ion-exchangecolumns. The water can be taken off under reduced pressure until theNMMO concentration has reached 60% by weight. This NMMO solution canthen be used again for preparing the spinning solution. The NMMO is thusvirtually completely recovered.

[0024] Depending on caliber, the finished tubes, at a glycerol contentof 20 to 22% by weight and a water content of 8 to 10% by weight, ineach case based on the total weight of the tube, have a weight of 30 to120 g/m², preferably 35 and 80 g/m². The weight per unit area generallyincreases with increasing caliber. The bursting pressure is likewisedependent on the caliber (small calibers have a higher burstingpressure). For a tube having a caliber of 16 mm, the bursting pressureis about 60 kPa, for a caliber of 30 mm about 40 kPa, at a caliber of 50mm about 24 kPa and at a caliber of 140 mm about 15 kPa. The burstingpressure is measured in each case here in the wet state.

[0025] The tubular casings according to the invention can, furthermore,be provided on the inside and/or outside with an impregnation orcoating, e.g. a liquid smoke impregnation or an “easy peel” internalpreparation. The same obviously applies to flat films.

[0026] An essential advantage of the flat or tubular films according tothe invention is the uniform structure and thus uniform density which isachieved on precipitation. Films which are produced by the viscoseprocess, in contrast, have a density gradient (higher density on thesurface, lower in the interior).

[0027] The tubular films according to the invention are preferably usedas sausage casings, in particular as “peelable casing” in the productionof frankfurters. In addition, they can also be used as membranes forvarious purposes, e.g. in hemodialysis. Finally, flat films can also beproduced by cutting open the tubes.

[0028] If the cellulose tubes are used as sausage casings, the stuffingcaliber can correspond to the annular gap diameter or up to 120% aboveit. Preferably, the stuffing caliber is 10 to 80% above the annular gapdiameter.

[0029] The following examples serve for more detailed description of theinvention. Percentages therein are percentages by weight, unless statedotherwise. Flat width, weight of the casing and thickness of the casingwall were determined under standard conditions (55% relative humidity;23° C.).

EXAMPLE 1

[0030] 510 g of ground wood cellulose (®Cellunier F from Rayonier)having a mean degree of polymerization of 535 (determined by the Cuoxammethod) was mashed in 5087 g of a 60% strength NMMO solution. The pH ofthe mash was then adjusted to 11 by NaOH. Under reduced pressure, withstirring and heating, water was then distilled off with increasingtemperature, until, at an NMMO content of 87.7%, based on the totalweight of water and NMMO, the monohydrate was present (recognizable by arefractive index of 1.4820). During this phase which lasted forapproximately 4 hours, the vacuum was kept at 10 to 16 mmHg. Afterstirring for a further 2 to 3 hours, the cellulose was completelydissolved at about 85 to 95° C. In order that relatively little water isevaporated, the vacuum was set to about 200 mmHg during this time. Therefractive index then ranged from about 1.4910 to 1.4930, whichcorresponds to a water content of 7.5 to 9%.

[0031] The spinning solution prepared in this manner was extruded at atemperature of 90° C. through an annular die at a gap diameter of 20 mmand a gap width of 0.5 mm. The tube first passed through an air sectionabout 10 cm in length at a velocity of 20 m/min. In the course of thisit was transversely stretched by air fed internally. It then passedthrough a spinning bath section of 3 m. The spinning bath comprised a14% strength NMMO solution which was cooled to 5° C. A solution of thesame composition was also introduced into the tube interior (“innerbath”). The tube was then laid flat at a guide roll in the spinning vat.The tube had been stretched transversely to the extent that its flatwidth after leaving the spinning vat was 30 mm. The edges showed nodamage.

[0032] The tube then passed through 4 precipitation vats each having 8guide rolls at the top and bottom, a bath depth of 1 m and an airsection of 2 m. At the end of the last vat water was introduced whichwas conducted in counter-current. At the outlet of the first vat, theNMMO content was kept in this manner at 12 to 16%. The temperatureincreased up to 60 to 70° C. in the last vat. After passing through thisprecipitation section, residues of NMMO were washed out of the tube in 4washing vats. The temperature in these vats was likewise 60 to 70° C.Finally, the tube was conducted through a plasticizer vat whichcomprised a 10% strength glycerol solution having a temperature of 60°C.

[0033] At a swelling value of 290%, the finished tube absorbed 21%glycerol. The flat width on leaving the glycerol vat was still 20 mm.The tube was then dried with hot air between 2 pinch-roll pairs. Thedryer had a plurality of zones of decreasing temperature. The zone atthe inlet had a temperature of 120° C., and that at the outlet 80° C.Subsequently, the tube was wetted until its water content was 8 to 12%(based on the weight of cellulose) and was wound up. The burstingpressure of this tube was 52 kPa, its static extension was 20.5 mm, andits swelling value was 165%. It was then wetted to 16 to 18% andgathered in sections (“shirred to form shirred sticks”).

[0034] The shirred sticks were stuffed with sausage emulsion on anautomatic stuffing machine (®FrankAMatic), scalded and smoked.Thereafter, the casing was peeled by an automatic apparatus. In thescalding and smoking behavior, this peelable skin was at least as goodas one produced by the viscose process.

EXAMPLE 2

[0035] A spinning solution as described in Example 1 was extruded at atemperature of 90° C. through an annular die 45 mm in diameter and a gapwidth of 0.7 mm. At a velocity of 20 m/min, the tube formed in thismanner passed through an air section of 15 cm in order then to beimmersed in the spinning bath. In the air section it was stretchedtransversely, as described above, with compressed air. The spinning bathhad a depth of 3 m and was filled with a 12o strength aqueous NMMOsolution which had a temperature of 5° C. Spinning bath solution wascharged into the interior of the tube. On leaving the spinning vat, thetube had a flat width of 56 mm. Its swelling value was 302%. It was theninflated with reinforcing air, so that the flat width increased again to66 mm. Before being wound up, it was wetted until the water content was14 to 16%, based on the weight of the dry tube. The glycerol content was20% at a total weight of 56 g/m². Swelling value was determined at 158%.The soaked tube had a bursting pressure of 30 kPa. Its static extensionat an internal pressure of 15 kPa is 44 mm.

[0036] Sections of this tubular film each having a length of 50 m wereshirred to form shirred sticks, which were then stuffed by an automaticstuffing machine using fine Mettwurst emulsion to a stuffing caliber of44 mm. The sausages were then matured in the usual way and smoked. Thecasings according to the invention showed in this case properties atleast as good as cellulose casings produced by the viscose process.

EXAMPLE 3

[0037] The spinning solution described in Example 1 was extruded at atemperature of 90° C. through an annular die having a die gap diameterof 26 mm and a die gap width of 0.6 mm. In contrast to the two precedingexamples, the die body was joined to a 50 cm-long metal pipe over whichthe extruded tube was conducted. At the top end of the pipe there weresituated orifices for feeding compressed air (“support air”) requiredfor the transverse stretching and the spinning bath solution. The airsection between die gap and surface of the spinning bath was 2.5 cm. Thespinning bath was filled with a 3° C., 14% strength aqueous NMMOsolution. The spinning speed was 20 m/min. Sufficient support air wasfed so that the tube had a flat width of 40 mm on leaving the spinningvat. Further treatment was then performed as described in Example 1.After leaving the plasticizer vat, the flat width was still 28 mm. Theswelling value of the plasticized tube was 286%. It was then dried inthe inflated state, wetted to 12 to 16% water content and wound up on aroll. The finished tube had a weight of 44 g/m². It comprised 22%glycerol (based on its total weight) and a swelling value of 165%. Itsstatic extension at an internal pressure of 20 kPa was 25.8 mm, and itsbursting pressure (measured in the wet state) was 42 kPa.

[0038] The tubes were shirred in sections to form shirred sticks andwere stuffed with sausage emusion to a stuffing caliber of 26 mm on anautomatic stuffing machine. After scalding and smoking in theconventional manner, the casing was peeled off on an automatic plant andthe sausages were then packed in cans. The sausage casings according tothe invention complied with all requirements at least as well as thecasings produced by the viscose process.

EXAMPLE 4

[0039] The spinning solution prepared in accordance with Example 1,which comprised about 9% cellulose, was homogeneously mixed, withstirring, with 3%, based on the weight of the cellulose, of a sucrosemonostearic acid/palmitic acid ester at a temperature of 95° C. It wasthen (at the same temperature) extruded through an annular die having adiameter of 20 mm and a gap width of 0.5 mm. At a velocity of 20 m/min,the tube first passed through a 10 cm long air section. In the course ofthis it was pressurized internally with compressed air and thustransversely stretched. Further production steps were identical to thosedescribed in Example 1.

[0040] The casings gathered in sections (“shirred sticks”) were thenplaced onto the automatic stuffing machine and stuffed with sausageemulsion. After scalding and smoking, the casing was peeled off by anautomatic apparatus. During scalding and smoking, the casing exhibited abehavior like the casings produced by the viscose process.

[0041] In the table below, the properties of a casing produced by theconventional viscose process termed “comparison”) are compared withthose of two casings according to the invention, the one (termed “A”)comprising no modifying compounds and having been produced according toExample 1, whereas the other (termed “Be”) comprising 3%, based on theweight of the cellulose, of the sugar ester mentioned in Example 4 andwas produced in accordance with this example. Comparison A B Flat width*(mm) 29.2 26 28 Weight* (g/m²) 43.8 47.3 53.1 Thickness* (μm) 40 55 45Glycerol content** (%) 20.5 20.7 20.5 Swelling value (%) 154 165 169Ultimate tensile strength***, 1.5 22.6 21.5 longitudinal (N/mm²)Elongation at break***, 32 29.8 36.5 longitudinal (%) Change in lengthafter wetting (%) longitudinal −0.9 −1.0 +1.0 transverse −1.2 −1.0 −1.0after drying again (%) longitudinal −2.5 −3.3 −0.3 transverse −4.0 −10.5−8.9 Bursting pressure*** (kPa) 48 52 50

EXAMPLE 5

[0042] The spinning solution prepared in accordance with Example 1 wasadmixed with 5%, based on the weight of the cellulose, of a copolymer ofmethyl vinyl ether and maleic acid monobutyl ester (molar ratio 1:1) inthe form of a 50% strength ethanolic solution and homogeneously mixed ata temperature of 95° C. The solution was then extruded through anannular die having a diameter of 40 mm and a gap width of 0.7 mm. Thetube passed at a velocity of 20 m/min through an air section of 15 cm,within which it was transversely stretched by compressed air. Thefurther manufacturing steps were identical to those described in Example1.

[0043] On leaving the spinning bath, the tube had a flat width of 66 mm,which corresponds to a diameter of 42 mm. On leaving the glycerol vat,the flat width was 56 mm, and the swelling value 302%. The tube was theninflated with compressed air, so that the flat width increased again to66 mm. Before being rolled up, it was wetted again until the watercontent was 14 to 16%. The glycerol content was 20%, based on the totalweight of the casing, and the swelling value was 158%. The wetted tubehad a bursting pressure of 30 kPa and a static extension of 44 mm at 15kPa.

[0044] Sections each 50 m in length were shirred to form shirred stickswhich were then mounted on an automatic stuffing machine. The casing wasthen mechanically stuffed with fine Mettwurst emulsion to a caliber of44 mm, matured and smoked. The use properties corresponded to those ofcasings which had been produced by the viscose process.

[0045] The ultimate tensile strength (longitudinal) was 70%, theelongation at break 50%, above that of a casing likewise produced by theNMMO process, but without addition of the modifying compound. Theshrinkage was 12% less than with the comparison material. The modifiedcasing could be peeled off more readily from meat sausage than thenon-modified casing.

EXAMPLE 6

[0046] The spinning solution prepared in accordance with Example 1 wasadmixed with 12%, based on the weight of the cellulose, of a copolymerof vinylpyrrolidone and 2-(dimethylamino)ethyl methacrylate (molar ratio1:1) and homogeneously mixed at a temperature of 98° C. The solution wasthen extruded at this temperature through an annular die having adiameter of 26 mm and a gap width of 0.6 mm. At a velocity of 20 m/min,the tube passed through an air section of 50 mm, within which it wastransversely stretched by compressed air, so that on leaving thespinning vat it had a flat width of 40 mm. The further manufacturingsteps were identical to those described in Example 1. After leaving theplasticizer vat, the flat width was 36 mm, and the swelling value 286%.

[0047] The tube was then dried in the inflated state, wetted to 12 to16% and rolled up. It comprised 22% glycerol and then had a swellingvalue of 165%. Its weight per square meter was 44 g. The burstingpressure (in the wet state) was determined as 50 kPa. The staticextension at 20 kPa was 25.8 mm. The ultimate tensile strength in thewet state was 60%, and the elongation at break was 45%, over that of anunmodified comparison material.

[0048] The tubes were shirred in sections to form shirred sticks, whichwere stuffed with sausage emulsion on an automatic stuffing machine to astuffing caliber of 26 mm, scalded and smoked. The casing was thenpeeled off by an automatic apparatus and the sausages were packaged incans.

EXAMPLE 7

[0049] Example 6 was repeated with the difference that not 12% but 20%of the copolymer of vinylpyrrolidone and 2-(dimethylamino)ethylmethacrylate were mixed with the spinning solution. The solution wasextruded through an annular die having a diameter of 40 mm and a gapwidth of 0.6 mm. At a velocity of 28 m/min, the tube passed through anair section of 12 cm in length, within which it was transverselystretched by compressed air. The further manufacturing steps wereidentical to those in Example 1.

[0050] On leaving the spinning vat, the tube had a flat width of 70 mm.In contrast to the other examples, the tube here did not pass through asoftener vat, and therefore was free of glycerol. After drying it waswetted until the water content was 14 to 16% and then wound up. Theswelling value was 142%, the bursting pressure 42 kPa and the staticextension at 15 kPa was 44 mm. The shirred tube sections could bestuffed without problems on an automatic stuffing machine.

EXAMPLE 8

[0051] The spinning solution prepared in accordance with Example 1 wasadmixed with 0.8%, based on the weight of the cellulose,polyvinylpyrrolidone K 70 (mean M_(w): 200,000) and stirred at atemperature of 95° C. until a homogeneous mixture had formed. Thesolution was then extruded through an annular die having a diameter of80 cm and a gap width of 0.7 mm. At a velocity of 30 m/min, the tubepassed through an air section 40 cm in length, within which it wastransversely stretched by compressed air. The further manufacturingsteps were identical to those described in Example 1, but the spinningbath comprised a 10% strength NMMO solution. The glycerol vat compriseda 7%. glycerol solution.

[0052] The tube was dried in the inflated state in such a manner thatthe flat width of 130 cm was not changed. After wetting to 14 to 16%water content, the tube was wound up. It was then cut on one edge. Theflat film obtained in this manner had a weight of 40 g/m², a thicknessof 35 μm, a glycerol content of 217%, a swelling value of 165%, anultimate tensile strength of 20 N/mm² in the longitudinal direction and16 N/mm² in the transverse direction, an elongation at break in thelongitudinal direction of 42% and in the transverse direction of 54%.

1. A seamless tubular cellulose-based film, which is obtainable byextruding a cellulose-, N-methyl-morpholine N-oxide- andwater-containing spinning solution through an annular die and treatingthe tubular film in an N-methylmorpholine N-oxide-containing aqueousspinning bath, which comprises transversely stretching the film by blowmolding in an air section between annular gap and surface of thespinning bath.
 2. The film as claimed in claim 1 , wherein the cellulosecontent in the solution is 7 to 15% by weight, preferably 9 to 12% byweight, in each case based on the total weight of the solution.
 3. Thefilm as claimed in claim 1 or 2 , wherein the cellulose has a meandegree of polymerization of 300 to 700, preferably 400 to
 650. 4. Thefilm as claimed in one or more of claims 1 to 3 , wherein the spinningsolution comprises 90.5 to 92.5% by weight of NMMO and 9.5 to 7.5% byweight of water, based on the total weight of the solvent.
 5. The filmas claimed in one or more of claims 1 to 4 , wherein the spinningsolution comprises 0.2 to 50% by weight, preferably 0.5 to 20% byweight, particularly preferably 1 to 15% by weight, of modifyingcompounds, in each case based on the weight of the cellulose.
 6. Thefilm as claimed in claim 5 , wherein the modifying compound is starch, astarch derivative or cellulose derivative, a sugar ester, alginic acidor an alginate, chitosan, carrageenan, vinyl alcohol, vinyl acetate, anacrylate, an ester of a sugar and fatty acids, the ester having an HLBvalue of 1 to 15, an ethoxylated or nonethoxylated fatty acid or saltthereof, wax, paraffin, polyvinylpyrrolidone, a copolymer ofvinylpyrrolidone and 2-(dimethyl-amino)ethyl methacrylate, a copolymerof methyl vinyl ether and maleic anhydride or a copolymer of methylvinyl ether and maleic acid monoalkyl ester.
 7. The film as claimed inone or more of claims 1 to 6 , wherein the spinning solution is extrudedat a temperature of 85 to 105° C., preferably 90 to 95° C., through anannular die having a gap width of 0.1 to 2.0 mm, preferably 0.2 to 1.0mm.
 8. The film as claimed in one or more of claims 1 to 7 , wherein thedistance between annular gap and surface of the spinning bath is 1 to 50cm, preferably 2.5 to 20 cm.
 9. The film as claimed in one or more ofclaims 1 to 8 , wherein the spinning bath is an aqueous solution whichcomprises 5 to 50% by weight, preferably 8 to 20% by weight, of NMMO andhas a temperature of 0 to 50° C., preferably 2 to 20° C.
 10. The film asclaimed in one or more of claims 1 to 9 , wherein the extruded tube,downstream of the spinning bath, further passes through in each case aplurality of precipitation and washing baths.
 11. The film as claimed inone or more of claims 1 to 10 , wherein it is treated with aplasticizer, preferably glycerol.
 12. The use of the film as claimed inone or more of claims 1 to 11 as sausage casing, preferably as peelablecasing.
 13. The use of the film as claimed in claims 1 to 11 as flatfilm after cutting open the tube in the longitudinal direction.